WO 2010/100594 A2 discloses a method and a system for processing images of at least one living being, including:
obtaining a sequence of digital images taken at consecutive points in time;
selecting at least one measurement zone comprising a plurality of image points, wherein
the step of selecting at least one measurement zone includes analyzing information based on pixel data of a plurality of image parts in at least one of the images, each image part including at least one image point, and selecting each measurement zone from contiguous parts determined to have similar characteristics; and
for each measurement zone, obtaining a signal representative of at least variations in a time-varying average value of a combination of pixel values at at least a number of the image points for use in determining at least one of a presence and frequency value of at least one peak in a spectrum of a signal corresponding to a frequency of a periodic physiological phenomenon.
The document further discloses several refinements of the method and the system. For instance, the use of photoplethysmographic (PPG) imaging is envisaged.
Photoplethysmographic approaches can be utilized in so-called pulse oximeters which are typically configured for monitoring a subject of interest, for instance for monitoring a patient's blood oxygen saturation. Frequently, mediate detection of (arterial) blood oxygen saturation is referred to as SpO2-measurement.
Recently, remote digital image-based monitoring systems for obtaining patient information or, physiological information of living beings in general, have been described and demonstrated.
As used herein, the term “remotely detected electromagnetic radiation” may refer to radiation components which are sent to a subject of interest from a radiation source and “reflected” by a skin portion of the subject of interest. Since reflection mechanisms in the subject's skin are rather complex and multi-dependent on factors such as wavelength, penetration depth, skin composition, vascular system structure, and further influencing parameters, terms such as “emitted”, “transmitted” and “reflected” shall not be understood in a limited way. Typically, a portion of incident radiation may be reflected at the skin's (upper) surface. Furthermore, a portion of incident radiation may penetrate the skin and pass through skin layers. Eventually, at least a portion of the incident penetrating radiation may be absorbed in the skin, while at least another portion of incident penetrating radiation may be scattered in the skin (rather than reflected at the skin's surface). Consequently, radiation components representing the subject of interest which can be captured by a sensor can be referred to as re-emitted radiation.
For remote monitoring and measurement approaches, the use of cameras has been demonstrated. Cameras may particularly involve video cameras capable of capturing sequences of image frames. Preferably, cameras capable of capturing visible light can be used. These cameras may comprise a certain responsivity characteristic which covers at least a considerable portion of a visible light range of the electromagnetic spectrum. As used herein, visible light shall be understood as the part of the electromagnetic spectrum which can be sensed by the human eye without further technical aids.
Remote subject monitoring (e.g., patient monitoring) is considered beneficial since in this way unobtrusive measurements can be conducted. By contrast, non-remote (contact) measurements typically require sensors and even markers to be applied to a skin portion of interest of the subject to be monitored. In many cases, this is considered unpleasant, particularly for long-term monitoring.
It would be therefore beneficial to provide for a system and a method for remote monitoring which further contribute to overcoming the need of obtrusive (contact) measurement.
In this connection, Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photoplethysmographic signals can be measured remotely with normal ambient light and rather conventional video cameras. However, for remote measurement, huge disturbances have to be expected. Disturbances may involve undesired relative motion between the subject of interest and the monitoring device. Furthermore, varying illumination conditions may adversely influence monitoring reliability and monitoring accuracy. Additionally, since image capturing sensors (e.g., cameras) typically may capture a field of view (e.g., corresponding to a frame size) in which the subject of interest and further surrounding objects are present at the same time, a region of interest has to be selected and should be tracked, if possible. Also for the subject of interest, indicative portions that contain the desired physiological information (e.g., skin portions) and non-indicative portions (e.g., hair and clothes) can be present. Moreover, a plurality of subjects (e.g., patients) can be present in a captured frame. While for obtrusive, tactile measurements these adverse disturbing influences can be minimized, remote, non-obtrusive approaches are facing huge challenges in this regard.
Given that signals of interest may be embedded or, so to say, hidden in slight skin color fluctuations, or even in slightest motion patterns, considerably low signal to noise ratios have to expected, considering the massive adverse impacts of disturbances and distortions which may corrupt the captured data.
In some fields of application, the signal to noise ratio may be even lower. This might be the case when the monitoring or measurement is eventually directed at the determination of derived vital signs information which basically has to be determined in a mediate way on the basis of signals that can be directly obtained from the captured data.